Fix API request timer regression

[dlon]

This fixes the issue introduced yesterday where API requests are incorrectly paused.

Also, some unit tests were added. 🎉

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This change is 

[MarkusPettersson98]

Reviewed 3 of 3 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved

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mullvad-api/src/availability.rs line 97 at r1 (raw file):

        let should_reset = {
            let mut inner = self.acquire();
            inner.resume_background();

The actual fix

Code quote:

inner.resume_background();

[Serock3]

Reviewed all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved